Method for illuminating particles for the purpose of forming their images

ABSTRACT

Peak power of a coherent light beam generated by a laser and injected into an optical fiber is chosen so that the spectrum of the light beam emanating from the fiber, for illuminating particles, includes Raman lines.

FIELD OF THE INVENTION

The present invention relates to a method for illuminating particles forthe purpose of forming their images. The method of the present inventionis particularly, although not exclusively, suitable for particle sizeanalysis of particles of all kinds, and in particular water droplets.

BACKGROUND OF RELATED ART

A method of this type is already known, for example from document EP-1183 518, in which a coherent light beam, preferably a pulsed light beam,is injected into an optical fiber, via one end of the fiber, and saidparticles are illuminated by the pulses of the light beam emanating fromthe other end of said fiber.

To implement such a method, a monomode optical fiber is chosen for thepurpose of preventing the occurrence of parasitic noise in said images.

However, to obtain contrasted sharp images of said particles, the energyinjected at each pulse of the coherent light beam must be high enough toallow said particles to be illuminated sufficiently to form theirimages. In such a case when the injected light energy is high, despitethe use of a monomode optical fiber, the images of said particlesexhibit parasitic background noise and a moiré effect that degrade theirsharpness.

SUMMARY OF THE INVENTION

The object of the present invention is to remedy these drawbacks.

For this purpose, according to the invention, the method forilluminating particles for the purpose of forming their images, in whichmethod a coherent light beam is injected into the core of a monomodeoptical fiber, at one end of said fiber, and said particles areilluminated by the light beam emanating from said core at the other endof said monomode optical fiber, is noteworthy due to the fact that thepeak power of said injected coherent light beam is chosen so that thespectrum of said emanating light beam has, alongside the wavelength ofsaid injected coherent light beam, lines of shifted wavelengthsgenerated by Raman conversion in the material of said core.

Specifically, the Applicant has found that, surprisingly andunexpectedly, and taking the contrary standpoint of a person skilled inthe art, who by technical reflex tends to preserve, by any means, thesingle frequency of the illumination light beam, it is possible toremove, from said images, the background noise and the moiré effect bydelivering sufficient energy into the optical fiber to cause Ramanconversion of the light beam while it propagates along said monomodeoptical fiber.

One a posteriori explanation of this beneficial effect of the Ramanlines could be the following:

-   -   impurities are present in the optics of the device for        illuminating the particles and of the camera for taking images        of the latter, so that, when these impurities are illuminated by        a high-energy coherent laser beam, they cause diffraction        phenomena with the appearance of interference effects, which        results in a background noise. In addition, the reflections off        the various optics cause interference fringes, which are        manifested by a moiré effect in the images; and    -   however, when the illumination beam is no longer a        single-frequency beam but, on the contrary, includes Raman        lines, the various diffraction signals from said impurities        resulting from the various Raman wavelengths mix together and        counteract one another, so that the sharpness of the particles        is improved.

Of course, the above explanation is merely a hypothesis, anyinvalidation of which would not prejudice the present invention.

As is known, the first Raman line is caused by the action of saidcoherent light beam on the material of the core of the optical fiber andhas a wavelength greater than that of said beam. Likewise, the secondRaman line is caused by the action of the first Raman line on thematerial of the core of the optical fiber and has a wavelength greaterthan that of said first Raman line, etc., an nth Raman line being causedby the action of the (n−1)th Raman line on the material of said core andhaving a wavelength greater than that of said (n−1)th Raman line (nbeing an integer).

Thus, the wavelength of the Raman lines progressively increases. Toavoid the inconvenience due to the increase in Airy spots on the imagesof said particles and, therefore, to prevent the quality of said imagesbeing degraded, it is advantageous to take measures to ensure that, insaid light beam emanating from said monomode optical fiber, there is noRaman line in the infrared or close to the infrared. This may beachieved by choosing the length of said monomode optical fiberappropriately.

To implement the method according to the present invention, it isadvantageous to use a known monomode optical fiber having a specificwavelength, that is to say a monomode optical fiber in which thediameter of the core is matched to the wavelength of the beam to betransmitted by the optical fiber and is an increasing function of saidwavelength. For example, such a monomode optical fiber matched to awavelength of 515 nm may have a core diameter of 3 μm, whereas anotherone, matched to a wavelength of 780 nm, may have a core diameter of 4.9μm.

With such monomode optical fibers with specific wavelengths, anotherfeature of the present invention is that said injected coherent lightbeam, having a wavelength of λ₁, is injected into the core of a monomodeoptical fiber having a specific wavelength matched to a wavelength λ₂ ofgreater than λ₁.

Thus, the area for injection of the coherent light beam into said coremay be increased and the power density per unit area injected at theinput of the optical fiber may be high enough, but without saturatingthe latter.

Of course, measures must be taken to ensure that, by using a corediameter larger than that matched to the wavelength λ₁, the propagationwithin the fiber remains monomode. This is generally satisfied when theλ₂/λ₁ ratio is at least approximately equal to 1.2.

In one particular way of implementing the method according to theinvention, λ₁ is equal to 532 nm and λ₂ is equal to 630 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures of the appended drawing will make it clearly understood howthe invention can be realized. In these figures, identical referencesdenote similar elements.

FIG. 1 shows schematically a particle size analyzer employing the methodaccording to the present invention.

FIG. 2 is a very enlarged schematic cross section of the optical fiberused in the analyzer of FIG. 1.

FIG. 3 shows schematically the injection of the coherent light beam intothe core of said optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

The analyzer illustrated schematically in FIG. 1 comprises a pulsedlaser 1, coupled to one end 2 a of an optical fiber 2 by means of acoupler 3. The other end 2 b of the optical fiber 2 illuminates a fieldoptic 4 that focuses the light beam emerging from the optical fiber 2onto the objective optic 5 of a camera 6, defining a measurement region7 in which particles 8 lie.

Said particles 8 may be stationary or passing through, possibly at highspeed, the measurement region 7, as is shown symbolically in FIG. 1 bythe arrow 14.

The laser 1 is, for example, of the frequency-doubled YAG (yttriumaluminum garnet)-type emitting, at a wavelength λ₁ equal to 532 nm forexample, a pulsed beam 9 (see FIG. 3).

The optical fiber 2 is of the monomode type with a specific wavelength.It comprises (see FIG. 2) a core 10, for example made of very puresilica, surrounded by a cladding 11, for example made of silica of lowerquality. The diameter of the core 10 is between a few microns and atmost about ten microns, whereas the diameter of the cladding 11 exceedsone hundred microns. In addition, the refractive index of the cladding11 is lower than that of the core 10.

In general, the monomode optical fiber 2 is matched to the monomodetransmission of a laser beam 9 of defined specific wavelength with,however, a few tolerances: thus, in the example given above of a laserbeam 9 operating at 532 nm, it would be possible to use a commerciallyavailable monomode optical fiber 2 especially constructed for themonomode transmission of a laser beam of wavelength equal to 514 nm.

In the coupler 3, the pulsed coherent beam 9 is focused by an optic 12onto the end face 10 a of the core 10, corresponding to the end 2 a ofthe fiber 2 (see FIG. 3).

Emerging at the opposite end 2 b of the monomode optical fiber 2, fromthe core 10, is the light beam 13 that illuminates the particles 8.

It is absolutely essential for the laser 1 to deliver, at each pulse ofthe laser beam 9, a high energy so that each pulse of the light beam 13illuminates said particles 8 sufficiently for the camera 6 to be able toform satisfactory images thereof.

For this purpose, the peak power of the laser beam 9 is chosen to behigh enough for, on the one hand, the particles 8 to be correctlyilluminated, so that the camera 6 can form their image, and, on theother hand, the spectrum of the illuminating light beam 13 to include,in addition to the wavelength of the laser beam 9, lines of shiftedwavelengths generated by Raman conversion in the core 10 in order,according to the invention, to remove the background noise and the moiréeffect from the images.

By reducing the length of the monomode optical fiber 2, the Raman linesin the infrared and close to the infrared, which are unnecessary anddetrimental, are removed.

Moreover, to facilitate the injection, without saturation, of the highenergy of the laser beam 9 into the end 10 a of the core 10, themonomode fiber 2 having a specific wavelength is matched to a wavelength2 greater than the wavelength λ₁ of the laser beam. Thus, the diameterof the core 10 is larger than if the monomode fiber 2 were strictlymatched to the wavelength λ₁.

In the above example, in which λ₁ is equal to 532 nm, λ₂ may be chosento be equal to 630 nm so that the λ₂/λ₁ ratio is approximately 1.2.

1. A method for illuminating particles for the purpose of forming theirimages, in which method a coherent light beam is injected into the coreof a monomode optical fiber at one end of said fiber, the other end ofthis optical fiber illuminates a field optic that focuses the light beamemerging from the optical fiber onto the objective optic of a camera,defining a measurement region in which said particles lie, and saidparticles are illuminated by the light beam emanating from said core forallowing the camera to form images thereof, wherein the peak power ofsaid injected coherent light beam is chosen so that, on the one hand,the particles are correctly illuminated so that the camera can formtheir image, and, on the other hand, the spectrum of the illuminatinglight beam includes, in addition to the wavelength of the laser beam,lines of shifted wavelengths generated by Raman conversion in the corein order to remove the background noise and the moiré effect from theimages.
 2. The method as claimed in claim 1, wherein the Raman lineswhose wavelength forms part of the infrared range and those whosewavelength is close to this range are removed from said spectrum.
 3. Themethod as claimed in claim 2, wherein said Raman lines corresponding tothe infrared or close to the infrared are removed by adjusting thelength of said monomode optical fiber.
 4. The method as claimed in claim1, in the implementation of which a monomode optical fiber having aspecific wavelength is used, that is to say a monomode optical fiber inwhich the diameter of the core is matched to the wavelength of the beamto be transmitted by the optical fiber and is an increasing function ofsaid wavelength, wherein said coherent light beam, having a wavelengthof λ₁, is injected into the core of a monomode optical fiber having aspecific wavelength matched to a wavelength λ₂ of greater than λ₁. 5.The method as claimed in claim 4, wherein the λ₂/λ₁ ratio is at leastapproximately equal to 1.2.
 6. The method as claimed in claim 4, whereinλ₁ is equal to 532 nm and λ₂ is equal to 630 nm.